Body made of a ceramic material

ABSTRACT

A body made of a ceramic material stabilized by a stabilizing agent and a process for preparing the body. The body includes a surface region extending from the surface of the body to a predetermined depth. The stabilizing agent is enriched in the surface region. The proportion of the stabilizing agent in the surface region continuously increases from the predetermined depth to the surface. The process includes applying the stabilizing agent to the surface of a basic body made of a ceramic material and heating the basic body with the stabilizing agent applied thereon at a temperature such that at least a portion of the stabilizing agent diffuses into the ceramic material.

This is a Continuation of application Ser. No. 13/574,643 filed Jul. 23, 2012, which in turn is a national stage of PCT/EP2011/001205, filed Mar. 11, 2011, which claims the benefit of EP 10 003 578.1, filed Mar. 31, 2010. The disclosure of the prior applications is hereby incorporated by reference herein in its entirety.

The present invention relates to a body made of a ceramic material stabilized by a stabilizing agent, according to the preamble of claim 1, to a process for preparing said body, according to the preamble of claim 10, and to the use of the body as an implant, in particular a dental implant.

Dental implants which are currently in use are in general made of a metal, e.g. titanium, or a ceramic, e.g. a zirconia based ceramic.

In contrast to metal implants, which are dark and therefore mismatch with the color of natural teeth, ceramic materials have the advantage that their color can be closely matched to the natural tooth color. Efforts have thus been made to provide dental implants, of which at least the parts that are visible after insertion are made of a ceramic material.

Despite their favourable properties with regard to the color, the use of ceramic materials for dental implants is in many cases limited by their fatigue stability, which is generally rather low.

A ceramic material having a high mechanical strength is disclosed in U.S. Pat. No. 6,165,925, which relates to an yttrium oxide-stabilized zirconium oxide in predominantly tetragonal form (yttria-stabilized tetragonal zirconia; Y-TZP) for the production of a sintered semi-finished article.

Despite its favourable mechanical properties, in particular its high strength, toughness and wear resistance, yttria-stabilized tetragonal zirconia (Y-TZP) has however a propensity to low-temperature degradation (LTD) in the presence of moisture, as for example described by Chevalier et al., J. Am. Ceram. Soc., 92 (9), 1901-1920 (2009).

Low-temperature degradation is a kinetic phenomenon in which polycrystalline tetragonal zirconia transforms to monoclinic zirconia over a rather narrow but important temperature range, typically from room temperature to about 400° C.

The degradation progresses from the surface of the material to its interior and is accompanied by micro- and macrocracking, thus resulting in reduced fracture strength of the material.

This problem, which is also referred to as “low hydrothermal stability”, is particularly relevant for the use of zirconia for dental implants, as thereby the material is exposed to a humid and warm environment and needs to fulfil relatively strict safety requirements over a long period.

In addition, dental implants are often subjected to a subtractive treatment in order to improve its osteointegrative properties. In this regard, EP-A-1 982 670, for example, relates to a process for providing a topography to the surface of a dental implant made of a ceramic material, wherein at least a part of the surface is etched with an etching solution comprising hydrofluoric acid. Etching of the ceramic material has however been found to often go along with a further deterioration of its hydrothermal stability.

For improving its hydrothermal stability, it has been suggested to dope yttria-stabilized zirconia with a suitable amount of ceria. In this regard, it is referred to Huang et al. Journal of the European Ceramic Society 25 (2005), pp. 3109-3115 and to Settu et al., Journal of the European Ceramic Society 16 (1996), pp. 1309 to 1318, both referring to yttria-ceria-stabilized zirconia.

Yttria-ceria-stabilized zirconia has however the disadvantage that it is darker in colour than yttria-stabilized zirconia. This is particularly disadvantageous if the material is used for a dental implant, which preferably has a light colour matching with the natural tooth colour. Further, yttria-ceria-stabilized zirconia has the disadvantage that it is not suitable for being subjected to hot-isostatic pressing (HIP) after sintering. Thus, the strength obtainable for yttria-ceria-stabilized zirconia is lower compared to yttria-stabilized zirconia.

Alternatively, a homogenous dispersion of Al₂O₃ grains into a tetragonal yttria-stabilized zirconia matrix has been reported to increase the hydrothermal stability of the tetragonal phase, as for example stated in the above mentioned article of Huang et al. However, also the doping with alumina has a negative impact on the translucency of yttria-stabilized zirconia.

Considering the shortcomings of known stabilized ceramic materials, and in particular yttria-stabilized zirconia, the problem of the present invention is thus to provide a body made of a ceramic material, said body having an improved hydrothermal stability, i.e. an improved long-term stability under warm and humid conditions, by maintaining the mechanical and visual properties of the ceramic material on which the body is based.

The object is achieved by the body according to claim 1. Preferred embodiments are given in the dependent claims.

The present invention thus relates to a body made of a ceramic material stabilized by a stabilizing agent. According to the invention, the body comprises a surface region extending from the surface of the body to a predetermined depth, the stabilizing agent being enriched in said surface region.

The surface region thus reaches from the surface down to a depth, which can vary depending on the specific purpose of the body.

The term “enriched” as used in the context of the present invention relates on the one hand to the case where in the surface region the proportion of stabilizing agent of the ceramic material is higher than in the remainder of the body. Since the stabilizing agent may also be enriched starting from the surface region having a lower proportion of stabilizing agent than the remainder of the body, the term “enriched” may also relate to the case wherein in the surface region the proportion of stabilizing agent is as high as in the remainder of the body.

The term “proportion” as used in the context of the present invention relates to the atomic percentage or molar percentage of the stabilizing agent in atomic or oxidic form, respectively, relative to the total number of atoms or molecules, respectively, of the ceramic material.

Since only the surface region is enriched in the stabilizing agent, the other mechanical properties of the ceramic material—apart from the hydrothermal stability—can be kept essentially unchanged. In particular, the strength, toughness and wear resistance of e.g. yttria-stabilized tetragonal zirconia can be maintained. Also, the visual appearance of the body can be kept essentially unchanged.

According to a preferred embodiment of the present invention, the surface region extends from the surface of the body to a depth of at least 20 nm, more preferably at least 50 nm, most preferably at least 100 nm, thus providing an especially high hydrothermal stability. In order not to interfere with the mechanical, visual and osteointegrative properties of the body, the surface region extends from the surface of the body to a depth of less than 10 μm (micrometer), more preferably less than 5 μm (micrometer), most preferably less than 1 μm (micrometer). The surface region thus preferably extends to a depth ranging from 20 nm to 10 μm, more preferably 50 nm to 5 μm, and most preferably from 100 nm to 1 μm.

As will be explained in detail below, enrichment of the stabilizing agent in the surface region of the body can be achieved by a very straightforward process comprising the steps of applying a stabilizing agent to the surface of a basic body and heating the basic body with the stabilizing agent applied thereon at a temperature such that at least a portion of the stabilizing agent diffuses into the ceramic material in which it is integrated. In the body obtained, the proportion of the stabilizing agent of the ceramic material typically increases continuously from the predetermined depth to the surface.

The enrichment in the stabilizing agent can be achieved by the same stabilizing agent or a different stabilizing agent than the one of the basic body. Preferably, the stabilizing agent applied is the same as the one of the basic body, as thereby no further component is introduced.

The basic body is typically prepared by a sintering process. Sintering processes for achieving a ceramic body, and in particular a ceramic dental implant, are known to a skilled person.

In order to provide osteointegrative properties to the body, which is of particular relevance if the latter is used as a dental implant, at least a part of the surface of the body has a surface roughness. As mentioned above, the process for providing a surface roughness typically includes etching of the surface, which might lead to a deterioration of the hydrothermal stability of the body. In this context, it is referred to Journal of the American Ceramic Society, 69 [7] 583-84 (1986), according to which in acidic solutions, the selective dissolution of yttrium components is accelerated because yttrium ion is more basic than zirconium ion. According to the present invention, the deterioration of the hydrothermal stability can be reversed by enriching the stabilizing agent in the surface region; the body is thus re-stabilized.

Given the fact that the stabilizing agent diffuses into the ceramic material and is thereby integrated in the material, there is no discrete coating and thus no discrete boundary between the stabilizing agent applied and the basic body. Consequently, there is no splitting off of the stabilizing agent applied, as it is typically seen when a separate coating of an additional material is applied on a ceramic body.

In principle, the invention relates to any ceramic material stabilized by a stabilizing agent. Specifically, the stabilizing agent is preferably selected from the group of yttrium, cerium and their respective oxides.

More specifically, the body of the present invention is preferably made of a ceramic material comprising yttria-stabilized zirconia. In general, the yttria-stabilized zirconia used is tetragonal in phase. As mentioned above, yttria-stabilized tetragonal zirconia has a very high strength, a high toughness and a good wear resistance. In addition, the material has a light colour which can be closely adapted to natural tooth colour.

Other stabilized ceramic materials, such as an alumina based ceramic material, to which the present invention also relates, are known to the skilled person. In this regard, the skilled person is also aware of stabilizing agents other than the ones mentioned above. For example, calcium, indium, lanthanum and/or scandium as well as their respective oxides can be used as a stabilizing agent, depending on the ceramic material used and the purpose to be achieved. In this regard, it is also referred to the article of Chevalier mentioned above, indicating e.g. magnesium as further stabilizing agent. Apart from magnesium or magnesium oxide, also e.g. calcium or calcium oxide can be used as a stabilizing agent. Further stabilizing agents include for example Ga³⁺, which is also mentioned in the article of Chevalier. All these stabilizing agents are encompassed by the term “stabilizing agent” used in the context of the present invention.

As will be shown in detail below, the crystal structure of the ceramic material in the surface region of the body according to the present invention is typically such that the proportion of monoclinic phase is at most as high as in the remainder of the body. More specifically, the proportion of monoclinic phase is preferably less than 20%. This is in contrast to known ceramic bodies having no surface region enriched in stabilizing agent, since the transformation from tetragonal to monoclinic phase typically begins at the surface of the body, as for example explained by the above mentioned article of Chevalier et al. A suitable method for determining the proportion of monoclinic phase is specified in the context of the examples given below.

Apart from the body defined above, the present invention further relates to a process for preparing the body, said process comprising the steps of applying a stabilizing agent to the surface of a basic body made of a ceramic material and heating the basic body with the stabilizing agent applied thereon at a temperature such that at least a portion of the stabilizing agent diffuses into the ceramic material.

As mentioned above, the basic body is preferably made of a ceramic material comprising yttria-stabilized zirconia, but in no way limited thereto. The basic body is typically prepared by a sintering process known to the skilled person.

In order to achieve a sufficient diffusion of the stabilizing agent into the ceramic material, a sufficient temperature below the sintering temperature is preferably chosen. The actual temperature depends on the specific ceramic material and the stabilizing agent used. A person skilled in the art who has become aware of the present invention knows how to set this temperature.

As mentioned above, osteointegrative properties, which are of particular relevance if the body is used as a dental implant, can be achieved by providing a surface roughness to the body. According to the present invention, the process thus preferably comprises the step of roughening at least a part of the surface of the basic body by a subtractive treatment before applying the stabilizing agent.

As also mentioned above, a body having specifically high osteointegrative properties is obtained, if the subtractive treatment comprises an etching step. It is thereby particularly preferred that the etching step is carried out with an etching solution comprising hydrofluoric acid at a temperature of at least 70° C. By this treatment, discrete grains or agglomerates of grains are removed from the bulk ceramic material, whereby a surface having recesses and cavities is formed, thus leading to a “microscopic” surface roughness. A detailed description of this etching step is given in EP-A-1982670, in particular paragraphs [0024] to [0030], [0060] to [0064] and [0079] to [0081], the disclosure of which is hereby incorporated by reference.

It is further preferred that the subtractive treatment further comprises a sand-blasting step prior to the etching step. Thereby, a “macroscopic” surface roughness is obtained, which in combination with the “microscopic” surface roughness mentioned above further contributes to the high osteointegrative properties of the body.

According to another preferred embodiment of the process of the present invention, the stabilizing agent is applied to the surface of the basic body by a sol-gel process, in particular a dip-coating process, by chemical vapour deposition, by physical vapour deposition and/or by ion implantation.

Regarding the applying of the stabilizing agent by a sol-gel process, it is hereby referred to Makishima et al., J. Am. Ceram. Soc., 69 (6), 1989, C-127-C-129, who describe the preparation of CeO₂—TiO₂ coatings by a sol-gel dip-coating process. The process is applicable to other stabilizing agents, such as yttria.

In analogy, it is further referred to the article of Settu et al. specified above, who describe a sol-gel process for preparing Y₂O₃—ZrO₂ and Y₂O₃—CeO₂—ZrO₂ powders by adding oxalic acid to the mixed solutions of the metal salts at room temperature and to Bourell et al., J. Am. Ceram. Soc., 76 (3), 1993, pp. 705-711, who describe the sol-gel synthesis of nanophase yttria-stabilized tetragonal zirconia using zirconium tetrachloride and hydrated yttrium chloride precursors.

Further, Miyazawa et al., J. Am. Ceram. Soc., 78 (2), 1995, p. 347-55 describe the dip-coating of a zirconia sol on a substrate.

Regarding the applying of the stabilizing agent in oxidic form, in particular yttria, by chemical vapour deposition (CVD), it is hereby referred to Zhang et al, Chem. Mater. 1999, 11, 148-153, who describe the preparation of yttrium-oxide thin films by catalyst-enhanced chemical vapour deposition using oxygen as carrier gas.

The disclosure of the mentioned articles by Miyazawa et al., Bourell et al., Settu et al., Makishima et al. and Zhang et al. is hereby incorporated by reference.

In view of the process described above, the present invention also relates to a body obtainable by said process.

According to a further aspect, the present invention also relates to a body made of a ceramic material stabilized by a stabilizing agent, characterized in that the body comprises a surface region extending from the surface of the body to a predetermined depth, the amount of monoclinic phase of the crystal structure of the ceramic material being reduced in said surface region.

Specifically, the present invention relates according to this aspect to a body, wherein in the surface region the proportion of monoclinic phase of the crystal structure is at most as high as in the remainder of the body.

As mentioned above, the object achieved by the present invention is particularly useful in the field of implantology, in particular in oral implantology. The present invention thus further relates to the use of the body as an implant, in particular a dental implant.

The present invention is further illustrated by way of the following examples:

EXAMPLES Preparation of Discs

Discs of yttria-stabilized zirconia (ZIOLOX® MZ111 HIP of CeramTec AG; AZP 2009-0315) having a thickness of about 1 mm and a diameter of about 5 mm were prepared by cutting a rod made of said material. The discs were then etched in an etching solution comprising hydrofluoric acid (40%) at 95° C. for 10 minutes.

Coating of Discs

The discs were then coated with cerium, cerium oxide, yttrium, and yttrium oxide, respectively, using gas flow sputtering. Gas flow sputtering was performed in a vacuum chamber having a volume of 80 liters and equipped with a gas flow sputtering source comprising metallic cerium or yttrium targets, respectively (purity>99.9%).

The substrate holder was movable in horizontal direction thus allowing the substrate, i.e. the discs, to be kept in an unexposed state while conditioning the sputtering source and further allowing an oscillation movement during sputtering in order to coat a relatively large area of the discs in a uniform manner. The substrate holder was further equipped with a ceramic radiant heater, thus allowing setting the discs to a predetermined temperature. The discs were arranged on the substrate holder using conductive pads commonly used in the field of scatter-electron microscopy.

The following commercially available devices were used:

Source generator: ENI DC-Generator DCG100, max. 10 kW Heat generator: Electronic Measurements power supply unit Movement controller: ISEL CNC-Controller C10C-E/A Gas flow controller: MKS Multi-Gas Controller 647B Temperature controller: KS 90-1 temperature controller

The process parameters were typically set as follows:

Sputter target cerium: purity>99.9% Sputter target yttrium: purity>99.9% Target dimensions: hollow cylinder (length: 60 mm; inner diameter: 40 mm)

Typically, the sputtering step can be divided into four steps, i.e. pre-heating, conditioning of the source, applying of the coating and cooling.

In this regard, the specific process parameters used for the sputtering of the discs with cerium, and yttrium, respectively, (metallic coating) and with ceria (cerium oxide) and yttria (yttrium oxide), respectively, (oxidic coating) is given in Table 1.

TABLE 1 Process Metallic Oxidic parameter Conditioning coating coating Argon gas 850 sccm 850 sccm 850 sccm flow Oxygen gas 0 sccm 0 sccm 5 sccm flow Operating 0.42 mbar 0.42 mbar 0.42 mbar pressure Source power — 200 W 200 W Substrate 800° C. 800° C. 800° C. temperature Bias voltage — — — Bias — — — frequency Duration of 30 min 3 to 200 s 3 to 200 s application of coating

Coatings having a thickness of 5 nm, 25 nm, and 125 nm have been prepared using a relatively low coating rate of less than 50 nm/s. The respective samples are given in Table 2.

TABLE 2 Duration of Coating application thickness Sample No. Coating of coating (nm) 23 Ce 150 125 24 Ce 30 25 25 Ce 6 5 27 CeO 6 5 28 CeO 30 25 29 CeO 150 125 34 Y 200 125 35 Y 40 25 36 Y 6 5 38 Y₂O₃ 200 125 39 Y₂O₃ 40 25 40 Y₂O₃ 6 5

All oxidic coatings were fully transparent. No thin layer interference was detected for these samples.

The appearance of the metal coatings obtained under non-oxidative sputtering conditions depended strongly on the post-oxidation in air and differed depending on the metal applied and the thickness of the coating. All cerium coated samples became fully transparent in air, while absorbance of the yttrium coated samples depended on the thickness of the coating.

The proportion of zirconium, yttrium and cerium atoms, respectively, in the surface of the samples was determined using XPS (X-ray photoelectron spectroscopy). A non-coated disc has been used as a reference example (Ref.). The results are given in Table 3.

TABLE 3 Pro- Pro- Pro- portion portion portion Coating Zr Y Ce Sample thickness (atomic (atomic (atomic No. Coating (nm) %) %) %) Ref. — — 96.7 3.3 — 23 Ce 125 0.5 0.0 99.5 24 Ce 25 3.6 0.5 96.0 25 Ce 5 13.7 0.0 86.3 27 CeO 5 33.3 1.1 65.6 28 CeO 25 6.1 0.4 93.5 29 CeO 125 1.4 0.2 98.4 34 Y 125 0.1 99.9 — 35 Y 25 0.7 99.3 — 36 Y 5 10.1 89.9 — 38 Y₂O₃ 125 0.0 100.0 — 39 Y₂O₃ 25 0.5 99.5 — 40 Y₂O₃ 5 11.6 88.4 —

Determination of Proportion of Monoclinic Phase

In order to determine the crystal structure's proportion of monoclinic phase, the coatings were further analysed by X-ray diffraction (XRD). A diffractometer of the type Bruker D8 GADDS having a fixed incidence angle of 10° and being equipped with a Co-anode (30 kV/30 mA) and a graphite primary monochromator was used. The X-ray beam was focussed on the sample using 500 μm monocapillary optics with a 300 μm aperture.

Analysis of the diffraction patterns was performed in correspondence with DIN V ENV 14273 according to the method of Toraya et al, Calibration curve for quantitative analysis of the monoclinic tetragonal ZrO₂ system by X-rays diffraction, J. Am. Ceram. Soc., 1984, 67: 119-21.

The proportion of monoclinic phase can thus be determined according to the following equation:

$m = \frac{{I_{m}\left( {- 111} \right)} + {I_{m}(111)}}{{I_{m}\left( {- 111} \right)} + {I_{m}(111)} + {I_{t}(101)}}$

whereby I_(m)(111) and I_(m)(−111) refers to the monoclinic main reflexes and I_(t)(101) refers to the tetragonal main reflex. The proportion of monoclinic phase in the surface area of the samples is given in Table 4:

TABLE 4 Proportion of Sample Coating monoclinic No. Coating thickness (nm) phase (%) Ref. — — 3.6 23 Ce 125 13.0 24 Ce 25 5.8 25 Ce 5 5.8 27 CeO 5 5.9 28 CeO 25 11.0 29 CeO 125 11.5 34 Y 125 4.1 35 Y 25 4.9 36 Y 5 4.9 38 Y₂O₃ 125 9.7 39 Y₂O₃ 25 4.4 40 Y₂O₃ 5 4.2

The relatively high proportion of monoclinic phase for samples No. 23, 28, 29 and 38 seems to be an artefact and can be explained by the presence of a further phase (presumably CeO₂ and Y₂O₃, respectively).

Thermal Treatment

The samples were thermally treated by heating them to a temperature of about 1250° C. for 3 hours. Heating was performed in an oven (of Mihm-Vogt GmbH & Co. KG). The temperature was set only slightly below the sintering temperature of the basic material (being at about 1280° C.) in order to achieve an as high as possible diffusion of the coating material into the material of the basic body.

The samples were then analysed using the methods outlined above, in particular XPS and XRD.

Regarding the XPS analysis of the thermally treated samples, the results are given in Table 5:

TABLE 5 Pro- Pro- Pro- portion portion portion Coating Zr Y Ce thickness (atomic (atomic (atomic Sample No. Coating (nm) %) %) %) Ref. T — — 88.4 11.6 — 23 T Ce 125 47.7 9.5 42.8 24 T Ce 25 75.0 12.4 12.6 25 T Ce 5 85.3 12.6 2.1 27 T CeO 5 87.1 11.8 1.1 28 T CeO 25 77.9 12.2 9.9 29 T CeO 125 51.5 9.9 38.6 34 T Y 125 54.7 45.3 — 35 T Y 25 78.7 21.3 — 36 T Y 5 83.8 16.2 — 38 T Y₂O₃ 125 68.3 31.7 — 39 T Y₂O₃ 25 79.5 20.5 — 40 T Y₂O₃ 5 84.5 15.5 —

The results can be explained by a thermally induced diffusion of cerium, yttrium and their oxides, respectively, into the ceramic material of the basic body.

While for the cerium and cerium oxide coated samples there is only a slight dependency of the coating thickness on the resulting proportion of Ce in the ceramic material, the effect is much more pronounced for the samples coated with yttrium and yttrium oxide, respectively.

The samples were further analysed using X-ray diffraction (XRD) in order to determine the proportion of monoclinic phase. The results of the analysis are given in Table 6.

TABLE 6 Proportion of Sample Coating monoclinic No. Coating thickness (nm) phase (%) Reference — — 0.6 T 23 T Ce 125 0.0 24 T Ce 25 0.0 25 T Ce 5 0.0 27 T CeO 5 0.0 28 T CeO 25 0.0 29 T CeO 125 0.0 34 T Y 125 0.0 35 T Y 25 0.0 36 T Y 5 0.6 38 T Y₂O₃ 125 0.0 39 T Y₂O₃ 25 0.0 40 T Y₂O₃ 5 0.7

Hydrothermal aging of the samples was simulated according to ISO 13356 for 5 hours at 135° C. in an autoclave (Systec DE-56). The samples treated according to this accelerated aging procedure were analysed by X-ray diffraction (XRD) as outlined above and by field emission scanning electron microscopy (FE-SEM).

The results of the XRD analysis are given in Table 7.

TABLE 7 Pro- Pro- Pro- portion portion portion of Coating Y Ce monoclinic Sample thickness (atomic (atomic phase No. Coating (nm) %) %) (%) Reference 3.3 — 34.5 A Reference — — 11.6 — 30.9 TA 23 TA Ce 125 9.5 42.8 6.3 24 TA Ce 25 12.4 12.6 13.5 25 TA Ce 5 12.6 2.1 26.4 27 TA CeO 5 11.8 9.9 27.6 28 TA CeO 25 12.2 38.6 14.7 29 TA CeO 125 9.9 — 5.3 34 TA Y 125 45.3 — 0.0 35 TA Y 25 21.3 — 1.9 36 TA Y 5 16.2 — 20.1 38 TA Y₂O₃ 125 31.7 — 0.0 39 TA Y₂O₃ 25 20.5 — 3.4 40 TA Y₂O₃ 5 15.5 — 26.1

As given in Table 7, a proportion of monoclinic phase of less than 20% is achieved for all samples which prior to the heating have a coating thickness of at least 25 nm.

Based on the results given in Table 7, the phase transformation behaviour of samples having a surface region enriched in cerium or yttrium, respectively, is further shown in the figures of which

FIG. 1 shows a graphical representation of the proportion of cerium in the surface of samples 23 TA to 29 TA as a function of the proportion of monoclinic phase; and

FIG. 2 shows a graphical representation of the proportion of yttrium in the surface of samples 34 TA to 40 TA as a function of the proportion of monoclinic phase.

As is apparent from the figures, the phase transformation behaviour is completely different between the yttrium enriched samples and the cerium enriched samples. Whereas for the yttrium enriched samples, there is a threshold in the yttrium proportion in the range of about 15 to 20% starting from which the proportion of monoclinic phase readily falls to a value below 5%, the cerium enriched samples show a strong effect at a relatively low proportion of cerium which is continuously decreasing with increasing the proportion of cerium.

For the cerium- or ceria-enriched samples, a proportion of monoclinic phase of less than 20% is achieved if the proportion of stabilizing agent is higher than about 5 atomic %, whereas for the yttrium or yttria-enriched samples a proportion of monoclinic phase of less than 20% is achieved if the proportion of stabilizing agent is higher than about 20 atomic %.

As mentioned above, yttrium or yttria is preferably applied on the basic body if the latter is made of yttria-stabilized zirconia. As given in the examples, the proportion of yttrium in the surface is thereby preferably in the range from 20 to 25 atomic percents, thus allowing for obtaining a fully stabilized zirconia ceramic body. 20 to 25 atomic percents of yttrium can be achieved by applying an yttrium or yttria coating prior to the heating, said coating having a thickness ranging from 20 to 30 nm. 

1. A body made of a ceramic material stabilized by a stabilizing agent, the body comprising a surface region extending from a surface of the body to a predetermined depth, the stabilizing agent being enriched in the surface region, wherein a proportion of the stabilizing agent in the surface region continuously increases from the predetermined depth to the surface.
 2. The body according to claim 1, wherein the proportion of stabilizing agent of the ceramic material in the surface region is higher than in the remainder of the body.
 3. The body according to claim 1, wherein the stabilizing agent is selected from the group consisting of yttrium, cerium, and their respective oxides.
 4. The body according to claim 1, wherein the surface region extends from the surface of the body to a depth of at least 20 nm.
 5. The body according to claim 4, wherein the surface region extends from the surface of the body to a depth of at least 50 nm.
 6. The body according to claim 5, wherein the surface region extends from the surface of the body to a depth of at least 100 nm.
 7. The body according to claim 1, wherein at least a part of the surface of the body has a surface roughness.
 8. The body according to claim 1, wherein the body is made of a ceramic material comprising zirconia.
 9. The body according to claim 8, wherein the zirconia is yttria-stabilized.
 10. The body according to claim 1, wherein a crystal structure of the ceramic material in the surface region is such that the proportion of monoclinic phase is at most as high as in the remainder of the body.
 11. The body according to claim 10, wherein the crystal structure of the ceramic material in the surface region is such that the proportion of monoclinic phase is less than 20%.
 12. A process for preparing the body according to claim 1, the process comprising applying a stabilizing agent to a surface of a basic body made of a ceramic material and heating the basic body with the stabilizing agent applied thereon at a temperature such that at least a portion of the stabilizing agent diffuses into the ceramic material.
 13. The process according to claim 12, further comprising roughening at least a part of the surface of the basic body by a subtractive treatment before applying the stabilizing agent.
 14. The process according to claim 13, wherein the subtractive treatment comprises an etching step.
 15. The process according claim 14, wherein the subtractive treatment further comprises a sand-blasting step prior to the etching step.
 16. The process according to claim 12, wherein the stabilizing agent is applied to the surface of the basic body by a sol-gel process.
 17. The process according to claim 16, wherein the stabilizing agent is applied to the surface of the basic body by a dip-coating process, by chemical vapour deposition, by physical vapour deposition, and/or by ion implantation.
 18. A body obtainable by the process according to claim
 12. 